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Electric Meat

Why is the nervous system different from skin, liver, kidneys, or spleen?  The fundamental building blocks of the nervous system and muscles are electrically active cells.   These cells process inputs and generate outputs.  

Neurons are individual cells that form the elemental unit of every nervous structure in the  body.  The brain, spinal cord, peripheral nerves, and autonomic nervous system are all made up of neurons, and of glial cells, which are supporting cells for neurons.

Although they differ somewhat both morphologically and physiologically depending on where they are found and what function they serve, all neurons have a basic similar structure, illustrated in the image below (click to enlarge).

1)  All neurons have a cell body, or soma.  This is where the cell's nucleus and many other organelles reside.  2)  Most neurons have many dendrites, branching out from the soma like the branches of a tree.  These dendrites carry information (in the form of electrical signals) from other cells in towards the soma.  3)  Most neurons have a single axon, which carries information (again, electrical signals) out from the soma so that it may eventually be converted into chemical signals (neurotransmitters) and delivered to other cells through specialized connections called synapses.  These other cells are typically other neurons, but some neurons synapse onto muscle cells.

Muscle cells are also electrically active, and in the case of skeletal muscle, they receive information from neruons telling them when to contract.  

So how do these electrically active cells work? 

The Resting Potential

Most cells in the body naturally have a voltage gradient across their membrane.  That is, if you were put an electrode inside the cell, and a second electrode in the extracellular fluid just outside the cell, you would find a measureable voltage.  Or in other words, cells can function as tiny batteries.

This voltage difference is called the membrane potential.  

Inside the cell, the concentration of potassium ions, [K+], is much higher than in the extracellular fluid, and the concentration of sodium ions, [Na+], is much lower than it is in the extracellular fluid.  There is also a higher concentration of proteins with net negative charge inside the cell than outside.  These factors help produce the membrane potential.  A full description of the mechanisms underlying the generation of the membrane potential are complex and beyond the scope of this course. For further reading, I recommend Principles of Neural Science by Kandel et al. (ISBN: 0071390111).

In neurons and muscle cells, the membrane potential while the cell is at rest is called the resting potential, and is about -70 mV (the cytoplasm is negative relative to the outside of the cell).

If something causes the membrane potential to become more negative, we say that the membrane is hyperpolarized.   Conversely, if something causes it to become less negative (closer to zero), the membrane is depolarized.

The Action Potential

Lots of cells have a membrane potential.  What makes nerve and muscle cells unique is that they are electrically active: their membrane potential varies, and the cells react in characteristic ways to changes in the membrane potential.  

Very small changes in voltage don't produce much in the way of interesting behavior from the neuron.  But when a neuron membrane is depolarized beyond its threshold potential, a chain reaction occurs in the cell membrane.  A wave of strongly depolarizing current starts at the point where the cell was depolarized, and runs down the neuron's axon, setting off other events.  

What makes these changes possible are voltage-gated ion channels: special membrane channel proteins that are voltage-sensitive.  There are many.  for the purposes of this course, the most important one to understand is the voltage-gated sodium channel.   

The voltage-gated sodium channel is a protein channel that lies in the cell membrane.  At the resting potential, it is closed, and no ions pass through it.  When the membrane is depolarized, it undergoes a conformational change, and allows sodium ions to pass through it.  This change is short-lived, however, and soon the protein undergoes a second conformational change, and becomes inactivated.  Before it will open again, the membrane must first return to the resting potential, and the channel must return to the closed state. 

The axon membrane is chock full of voltage-gated sodium channels.  When something depolarizes the axon membrane, some voltage-gated sodium channels open up, and sodium ions begin to flow through.  Specifically, they flow in to the cell.  This happens because the concentration of sodium is much higher outside the cell than inside, and the sodium ions flow down this concentration gradient.  Another reason this happens is that the positive ions flow toward the negative interior of the cell along the voltage gradient.  

Whenever there is a net flow of charged particles, there is an electrical current.  And this electrical current results in a voltage change: the cell membrane is further depolarized.  If the membrane is depolarized enough (above the threshold potential), other nearby voltage-gated potassium channels will open, and increase the influx of sodium ions, further depolarizing the membrane.  The result is a chain reaction, causing a wave of strongly depolarizing current to flow down the entire length of the axon membrane.  This wave of depolarization is called the action potential, and it is crucial to the functioning of nerve and muscle cells.  It allows neurons to carry information long distances without degradation.  In a muscle cell, it is what triggers the events that cause the cell to contract.

There are other important voltage-gated ion channels.  For example, the voltage-gated potassium channel also plays a role in the action potential.  For the curious, I again refer you to Principles of Neural Science by Kandel et al. (ISBN: 0071390111).

Synapses 

Information is carried from one part of a neuron to another part of the same neuron by electrical signals, as described above.  But when it comes to communicating with other cells, the information is ususally sent by chemical signals.

Neurons usually receive information from other cells in the form of chemicals called neurotransmitters.  And in turn, and they usually deliver information to other cells by releasing neurotransmitters onto those cells.  

Neurons connect with other cells at structures called synapses.  

Terminology:

By the end of this section, make certain that you understand what each of these terms mean, and can apply them appropriately.  If applicable, make sure you can find each item on a photomicrograph, illustration, a whole brain, brain section, or image of a brain.

• neuron
• axon
• dendrite
• soma
• neurotransmitter
• synapse
• myelin
• glia
• gray matter
• white matter
• cortex
• ganglia
• nuclei
• tract
• nerve
• fasciculus